Amino acids as the building blocks of proteins, the formation of the peptide (amide) link by condensation, the hydrolysis of proteins to amino acids, essential amino acids, and how heat denatures the structure of enzymes.

What this key area is asking

The SQA wants you to describe amino acids as the building blocks of proteins, explain how the peptide link forms by condensation and breaks by hydrolysis, define essential amino acids, and explain how heating denatures an enzyme. The condensation-and-hydrolysis pair and the denaturing explanation are regular exam earners.

Amino acids

The peptide link

When the amino group of one amino acid reacts with the carboxyl group of another, a condensation reaction forms the peptide link (an amide link, $-CONH-$) and releases a molecule of water. The $-OH$ of one acid's carboxyl group and an $-H$ from the other's amino group leave together as that water molecule. Many amino acids joined this way form a long protein chain, and because each amino acid has both an amino group at one end and a carboxyl group at the other, the chain can keep growing in either direction.

Hydrolysis of proteins

During digestion, the body hydrolyses dietary proteins, breaking the peptide links with water to release the individual amino acids. Each peptide link broken consumes one molecule of water, the exact reverse of the condensation that formed it. The released amino acids are then absorbed and used to build the proteins the body needs, including its own enzymes, muscle and structural proteins.

Worked example: amino acids from hydrolysis

Enzymes and denaturing

Enzymes are proteins that work as biological catalysts. Their function depends on a precise three-dimensional shape, including the shape of the active site where the substrate binds.

When an enzyme is heated, it is denatured: the heat changes its shape so that the substrate no longer fits the active site, and the enzyme stops working. This is why cooking, fever or boiling destroys enzyme activity, and the change is usually permanent.

Examples in context

The chemistry of proteins is the chemistry of food and of the body. Cooking an egg is a vivid example of denaturing: the heat unfolds the proteins in the egg white, which then tangle together and set, an irreversible change in shape rather than a breaking of peptide bonds. Biological washing powders contain protease enzymes that hydrolyse the peptide links in protein stains such as blood and grass, breaking them into soluble amino acids, which is why such powders work best at warm rather than boiling temperatures, where the enzymes themselves would denature. Nutritionists rank dietary proteins by their content of the essential amino acids, since a protein lacking one of them, like some plant proteins, must be combined with others to give a complete supply.

Try this

Q1. Name the link formed when two amino acids join together. [1 mark]

• Cue. The peptide link (also called the amide link).

Q2. Explain what happens to an enzyme when it is heated strongly. [2 marks]

• Cue. It is denatured: heat changes its three-dimensional shape so it can no longer catalyse the reaction.

Q3. Calculate the number of moles in $9.00 \text{ g}$ of the amino acid glycine ($GFM = 75.0 \text{ g}$). [1 mark]

• Cue. $n = 9.00 / 75.0 = 0.120 \text{ mol}$.